The use of glass, glass-ceramic, and ceramic bodies in food service applications, i.e., as culinary ware and tableware, is widely recognized. Such bodies, especially those prepared from glasses and glass-ceramics, have occasionally been marketed in the plain state, i.e., without external decoration. Nevertheless, the great majority of the commercially-available food service products relies upon the aesthetic appearance of the products to promote customer appeal. Consequently, research and development have been quite extensive to provide decorative designs and finishes for the surfaces of such bodies. The bulk of this effort has been directed to developing attractive glazes and enamels. Reduced to the most fundamental terms, glazes can be considered clear glasses and enamels as glazes containing pigments to impart color thereto. Both products are applied to the surface of the glass, glass-ceramic, or ceramic body to be decorated in the form of very finely-divided particles, customarily referred to as "frit", and this frit then fired to fuse to and form a strongly-adherent, continuous film on the surface of the body.
Up to the present time most of the commercially-marketed frits for decorating glass, glass-ceramic, and ceramic ware have included substantial amounts of lead oxide (PbO) and, in several instances, cadmium oxide (CdO). Those compounds have been incorporated into the frit compositions for two principal reasons: (1) to soften the frit, i.e., to lower the melting point thereof so it could be fused onto the surface of the ware at a sufficiently low temperature that thermal deformation of the ware could be avoided; and (2) to raise the refractive index thereof. CdO has also been utilized as a colorant in certain frits. Cadmium and lead, however, possess the inherent disadvantageous characteristic of being highly toxic, such that glazes and enamels containing those metals which come into contact with food must display extremely good resistance to attack by acidic and alkaline materials, thereby preventing the release of any significant amount of those metals. The danger arising from the release of those metals, when compositions containing such come into contact with food, has been recognized by the Food and Drug Administration (FDA). That agency, in its Compliance Guidance Manual, issued June 13, 1974, established maxima for cadmium and lead release which a food-contacting surface must not exceed.
It is quite apparent that the potential for cadmium and/or lead toxicity could be completely removed by preparing frits wherein those metals were totally absent. The inability to satisfy the following combination of four criteria demanded in an acceptable glaze or enamel has forestalled the simple elimination of those metals, particularly lead, from the frits.
First, the firing or maturing temperature of the frit, viz., the temperature at which the frit will flow sufficiently to produce a smooth homogeneous coating, must be low enough to preclude thermal deformation of the ware being coated.
Second, the coefficient of thermal expansion must be compatible with that of the ware to inhibit crazing and/or spalling of the resultant glaze or enamel. Most advantageously, the frit will demonstrate a coefficient of thermal expansion somewhat less than that of the ware being coated. Such practice places the decorating coating in a state of compression with respect to the ware after being fired thereon.
Third, the frit must exhibit excellent resistance to attack by acids and bases and, in food service applications, to attack by detergents used in commercial dishwashers. Corrosion of the surface can result in loss of gloss, the development of haze or iridescence in the glaze or enamel, the formation of porosity, or other effects deleterious to the aesthetic appearance or physical character of the decorative coating.
Fourth, the frit must demonstrate good glass stability, i.e., the frit must resist devitrification during the firing operation.
Although not mandatory, a fifth desirable property for the frit is a refractive index high enough to yield a coating displaying a clear high gloss.
Whereas compositions have been explored which have satisfied several of those criteria, research has been constant to develop frits free from lead and cadmium which would answer the full combination of those requirements.
It will, of course, be appreciated that frits can be utilized for purposes other than as bases for decorative glazes and enamels. In such applications a high gloss, resulting from the high refractive index of the composition, may not be necessary. Also, in certain decorative practices, a highly glossy surface coating is not deemed desirable. Nevertheless, under either of the circumstances, the frits will still advantageously display the first four of the above-cited criteria. Thus, the firing temperature of the frit will be low enough to avoid thermal deformation of the substrate being coated. The coefficient of thermal expansion of the frit must be compatible with that of the substrate. The frit must demonstrate excellent chemical durability. And, to insure the development of a transparent coating, the frit will exhibit good glass stability.